Field of the Invention
The present invention relates to a display device and, more particularly, to a display device having a touch sensing system.
Discussion of the Related Art
A variety of types of flat display devices capable of reducing weight and volume (which are disadvantages of the cathode ray tube) are being developed. Such flat display devices may include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), and an electroluminescence (EL) device.
It is desirable that portable or mobile devices use display devices that are thin, light, and have low power consumption. Thus, flat display devices are often used as the display means for mobile communication terminals or handheld information processing devices, because they are thin and light in weight.
Furthermore, recent years have seen the research and development of a touch screen panel in which a user inputs information by directly touching a screen using, for example, a finger or conductive substance. The touch screen panel may be advantageous by not necessarily requiring an additional input device, and contents displayed on a screen may be easily and rapidly manipulated through the mere touch of a finger. A touch screen panel having such advantages has been used for various display devices, such as portable information devices (e.g., smart phones), and home appliances (e.g., refrigerators in which display units are formed).
The touch screen panel may be a resistive type, in which metal electrodes are formed in an upper plate or lower plate and a touched location is determined in the form of a voltage gradient according to resistance when a DC voltage is applied, a capacitive type, in which an equipotential is formed in a conductive layer and a touched part is detected by sensing the location where the voltage of an upper/lower plate is changed by a touch, or an electromagnetic type, in which a touched part is detected by reading an LC value induced when an electron pen comes in contact with a conductive layer, depending on a method of detecting a touched part.
The capacitive type touch screen panel may be a self-capacitance type or a mutual capacitance type. In a touch screen panel of a mutual capacitive type, a touch input may be detected by sensing a change in capacitance, e.g., a change in electric charge, when a finger or conductive substance comes in contact with the touch screen panel.
An example mutual capacitance type touch screen panel is described below with reference to FIGS. 1 and 2. FIG. 1 is a plan view schematically illustrating a mutual capacitance type touch screen panel, and FIG. 2 is a schematic cross-sectional view taken along line I-I′ of FIG. 1.
With reference to FIGS. 1 and 2, the mutual capacitance type touch screen panel may include touch driving electrodes Tx, touch sensing electrodes Rx, and a base film BF interposed between the touch driving electrodes Tx and the touch sensing electrodes Rx.
The touch driving electrodes Tx are formed on one surface of the base film BF, and are arranged in parallel in a first direction (e.g., an X-axis direction). The touch sensing electrodes Rx are formed on the other surface of the base film BF, and are arranged in parallel in a second direction (e.g., a Y-axis direction) crossing the touch driving electrodes Tx. The touch driving electrodes Tx and the touch sensing electrodes Rx are arranged to cross each other and are insulated from each other by the base film BF (including an insulating substance) interposed therebetween. The touch driving electrodes Tx and the touch sensing electrodes Rx overlap each other with the base film BF interposed therebetween to form a mutual capacitance Cm.
A sensing method of the mutual capacitance type touch screen panel, configured as described above, is described briefly below. Electric charges are supplied to the mutual capacitance Cm by applying a driving voltage to the touch driving electrodes Tx. In this case, an external touch input can be detected by sensing a change of capacitance through the touch sensing electrodes Rx in synchronization with the driving signal.
In the mutual capacitance type touch screen panel described above, the initial value of the mutual capacitance Cm may be great because the touch driving electrode Tx and the touch sensing electrode Rx have a wide overlap area. If the initial capacitance (Cm) value is great, there may be a problem of it being difficult to distinguish a touch from a non-touch, because a change of capacitance anterior and posterior to a touch is small. Furthermore, if the initial capacitance (Cm) value is great, an RC delay in touch sensing is generated because the time constant is increased. The RC delay may reduce a change of capacitance anterior and posterior to a touch as described above. In order to solve such problems, a sensing frequency may be lowered by taking the RC delay into consideration. However, as the sensing frequency is lowered, there may be a problem in that touch sensitivity is deteriorated because a touch report rate, that is, a rate in which the coordinates of each touch input are transmitted, is decreased.